State the expression for apparent frequency when source of sound and listener are

i. Let, 

n = actual frequency of the source. 

n₀ = apparent frequency of the source, 

v = velocity of sound in air. 

v_s = velocity of the source. 

v_l = velocity of the listener.

ii. Apparent frequency heard by the listener is given by,

\(n = n_0(\frac{v \pm v_L}{v \pm v_s})\)

Where upper signs (+ ve in numerator and -ve in denominator) indicate that source and observer move towards each other. Lower signs (-ve in numerator and +ve in denominator) indicate that source and listener move away from each other.

iii. If source and listener are moving towards each other, then apparent frequency is given by,

\(n = n_0(\frac{v + v_L}{v - v_s})\) i.e., apparent frequency increases.

iv. If source and listener are moving away from each other, then apparent frequency is given by,

\(n = n_0(\frac{v - v_L}{v + v_s})\) i.e., apparent frequency decreases.